Reactively decoupled dual channel keying circuit for wide-band frequency modulated keyable control circuit

ABSTRACT

A keyable control circuit, has sensing coils located in the vicinity of a plurality of locations where lock control is desired. At least one swept high-frequency oscillator, which is connected to each sensing coil, generates an rf signal, rapidly swept over a wide frequency band. When an external keying circuit, containing more than one resonant circuit, each correctly tuned to a predetermined keying frequency, is inductively coupled to a sensing coil, each resonant circuit absorbs rf energy as the oscillator frequency is swept past its resonant frequency. Electrical interaction between the resonant circuits is accomplished by reactive cancellation. Energy absorption in the external keying circuit induces corresponding reductions in rf energy in the sensing coil as the oscillator frequency is swept past the keying frequencies. Tuned detectors within the keyable control circuit produce a control signal when energy reduction is sensed at each of the predetermined keying frequencies. If correct absorption fails to occur at any one or more of the predetermined frequencies, the control signal is withheld. 
     A time-gating system enables selective direction of the control signal to one or more using locations while excluding others. This function finds convenient application in automotive use where it is frequently desired to enable simultaneous unlocking of one or more doors while excluding unlocking of the deck lid. 
     A dead-oscillator detector averts attempted actuation of the unlocking function by the coupling of untuned energy-absorbing material, such as iron, to a sensing coil.

BACKGROUND OF THE INVENTION

A number of patents disclose single-channel keyable control circuits.For example circuits disclosed in U.S. Pat. Nos. 3,624,415 and3,628,099, both in the names of Carl E. Atkins and Arthur F. Cake, showkeying circuits which require that the correct value of resistance in anexternal keying circuit be connected to actuate a keyable controlcircuit. In U.S. Pat. No. 3,723,967 in the names of Carl E. Atkins andPaul A. Carlson, a single channel inductively coupled tuned keyingcircuit absorbs energy from the radio frequency tank circuit of afree-running oscillator operating at the frequency to which the keyingcircuit is tuned. Radio frequency detection circuits detect thereduction in energy remaining in the oscillator and thereupon produce acontrol signal.

In U.S. Pat. No. 3,842,324 an external keying circuit includes a diodehaving a sharply variable junction capacitance with changes in diodebias as a component in a tuned circuit. When coupled to a keyablecontrol circuit operating in the correct frequency range, absorbed rfenergy causes rapid cyclic fluctuations in diode bias. The resultingrapid fluctuations in keying circuit resonant frequency alternatelybring the keying circuit into and out of resonance with the rf frequencybeing generated. When in resonance, the keying circuit absorbs more rfenergy from the rf oscillator than when out of resonance. The resultingamplitude modulation in the rf oscillator is detected to provide acontrol output signal.

Single-frequency keyable control systems suffer from the fact that asimple detection device disclosed to a tamperer the frequency at whichhe must operate to actuate the unlocking mechanism. In fact, a tuneableabsorption wavemeter, which is the simplest type of frequency measuringdevice would itself activate the pure absorption unlocking mechanism inU.S. Pat. No. 3,723,967. A fixed frequency system, operating at two ormore frequencies simultaneously or in sequence, although increasing thedifficulty, similarly suffers from the ability of a tamperer to detectthe operating frequencies.

It is desireable to fabricate all resonant circuits of the keyingcircuit in a small unitary object such as a key fob, ring or watch-bandof the type which someone would habitually carry on his person. Thephysically desireable small size of the keying circuit normally producesthe electrically undesireable result of interaction between the resonantcircuits within. Such interaction tends to broaden and/or shift theresonant characteristics of the tuned circuits. Broadening of theresponse has the undesireable effect of reducing the number of useablekeying frequencies for a given frequency sweep as well as reducing themagnitude of rf absorption at the peak.

SUMMARY OF THE INVENTION

The instant invention uses two or more swept rf oscillators gated intooperation one at a time. One of the swept rf oscillators providesexcitation signals to one or more sensing coils located at one type ofload. Other swept rf oscillators provide excitation signals to othersensing coils for other types of loads. All of the swept oscillatorsreceive a cyclically varying sweep voltage from a single sweepgenerator.

When a keying circuit, containing two or more resonant circuits tuned tospecific keying frequencies within the oscillator sweep range, iscoupled to one of the sensing coils, detection circuits within thekeyable control circuit detect the depletion of rf energy from theoscillator at these specific keying frequencies. When rf energydepletion is simultaneously detected at all keying frequencies, anoutput circuit generates a control output signal. The absence of rfabsorption at any one specific keying frequency is sufficient to causethe control output signal to be withheld. The tendency toward inductivecoupling between two of the resonant circuits is cancelled by adecoupling reactance which applies an inverted sample of the couplingenergy to each of them.

Iron absorbs rf energy strongly and approximately equally over a widefrequency range. A piece of iron coupled to a sensing coil could thussignificantly reduce the rf energy at all of the specific keyingfrequencies. A dead-oscillator detector averts spurious generation of acontrol output signal due to broad-band energy absorption of a deadoscillator. The dead-oscillator detector requires that significant rfenergy be present at some frequencies within the rf sweep range beforeit will enable the control output signal to be generated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of one embodiment of the present invention.

FIG. 2 contains a schematic diagram of the embodiment shown in FIG. 1.

FIG. 3 shows a block diagram of the mutual coupling.

FIG. 4 shows a schematic diagram of a second embodiment of the keyingcircuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the block diagram shown in FIG. 1, when a correct keyingcircuit, shown generally at 10, is brought into inductive coupling withone of the sensing coils L1, L2 or L3 of a keyable control circuit,shown generally at 12, the keyable control circuit 12 generates one ormore lock control output signals 14, 16.

A swept deck lock control oscillator 18 feeds rf energy to itsoscillator tank circuit composed of deck sensing coil L3 and capacitorC4. The deck oscillator tank circuit L3, C4 is located at the sensinglocation. A shielded cable 20 connects rf energy from the deck lockcontrol oscillator 18 to the deck tank circuit L3, C4. The capacitanceand inductance of the shielded cable 20 as well as stray couplingbetween the deck sensing coil L3 and nearby objects combine with thecircuit values of L3 and C4 to determine the deck lock controloscillator 18 frequency.

A similar swept door lock control oscillator 22 feeds rf energy inparallel to first and second door tank circuits L1, C2 and L2, C3located adjacent to first and second vehicle doors respectively. As inthe deck arrangement previously described, the impedance of the shieldedcables 24, 26 from the sensing locations to the oscillator 22 plus straycoupling combine to determine the door lock control oscillator 22frequency.

A sequencing generator 28 alternately gates the two oscillators 18, 22into operation. The sequencing generator 28 also performs output gatingas will be explained later.

A sweep generator 30 provides a sweep voltage signal 32 in parallel tothe swept oscillators 18, 22. The sweep voltage is preferably oftriangular or sawtooth waveform but could be of sinusoidal or otherwaveform. The applied sweep voltage signal 32 causes the frequency ofwhichever oscillator is gated on at any instant to vary in step with thesweep voltage signal 32. The frequency sweep is very wide compared tothe means oscillator frequency. For example, and not as a limitation, afrequency sweep from 6 to 7 megahertz has been found feasible with thepractical circuit component specified later.

A first detector 34 tuned to a first frequency F1 receives inputs fromall tank circuits. A similar second detector 36 tuned to a secondfrequency F2 also receives inputs from all tank circuits.

For the purpose of the discussion which follows, assume that thesequencing generator 28 has enabled the deck lock control oscillator 18.The events leading to the generation of the deck lock control signal 16will be described. Since operation of the door lock control oscillator22 is essentially similar, its operation will not be described indetail.

The deck lock control oscillator 18 provides a widely swept frequencysignal to the deck sensing coil L3 resonated by parallel capacitor C4. Asample of the rf energy in the deck lock control oscillator 18 isconnected in parallel to first detector 34 and second detector 36.

The keying circuit 10 consists of two LC tuned circuits integrated intoa single electrical and mechanical assembly as shown in generalized formin FIG. 3. Due to the proximity of resonant circuit LC1 to LC2,undesireable mutual inductive coupling Z_(M) between the two circuitscan occur. This mutual inductive coupling Z_(M) causes the two resonantcircuits to interact with each other especially when the resonantfrequencies are close together thus making tuning erratic and keyingperformance unreliable. One solution consists of providing widefrequency separation between the resonant frequencies. Unfortunately,wide frequency separation significantly reduces the number of useablekeying frequency pairs.

The effect of undesired mutual coupling can be reduced or eliminated byfeeding a decoupling signal from one LC circuit to the other. Thedecoupling signal, shown being fed from LC1 to LC2 by decouplingreactance Z_(D) must be of the correct amplitude and phase to cancel thesignal induced in LC2 by the mutual coupling Z_(M). With properconnections, the decoupling reactance Z_(D) can be either an inductanceor a capacitance however an inductance is better able to maintain thecancellation at the two resonant frequencies.

An example of a keying circuit 10 employing inductive decoupling isshown in the keying circuit of FIGS. 1 and 2. A first LC tuned circuitin the keying circuit 10, comprised of inductor L8 and capacitor C37, isresonant at a different frequency from a second LC tuned circuitcomprised of inductor L9 and capacitor C38. Both LC tuned circuits areresonant at frequencies within the sweep range of the deck lock controloscillator 18. Decoupling inductor L10 is in series to a commonconnection with each of inductors L8 and L9. Circulating rf currents inL10 due to L8 are induced into L9 due to the sharing of L10 by L8 andL9. The rf current induced into L9 by decoupling inductor L10 isopposite in phase to the rf current induced into L9 by direct inductivecoupling from L8. When the size of L10 is properly chosen consideringthe sizes, spacing and orientation of inductors L8 and L9, substantialcancellation of the direct inductive coupling between L8 and L9 isachieved. One combination of inductances for L8, L9 and L10 which hasyielded substantially reduced interaction is included in the parts listappended hereto. Experimental determination of other combinations isreadily done by one skilled in the art. The cancellation of inductivecoupling between the resonant circuits avoids the broadening of theresponse curve and the consequent reduction in the number of useablekeying frequencies.

An example of a keying circuit 10 employing capacitive decoupling isshown in FIG. 4. Note that in this circuit, inductors L8 and L9 arewound in opposite directions. The outboard ends of L8 and L9 areconnected together. Thus decoupling capacitance Z_(C) is capable offeeding a decoupling signal of correct amplitude and phase to cancel themutual inductance between L8 and L9. The value of Z_(C) is small. It hasbeen shown in the laboratory that circuit physical arrangements can beachieved in which the capacitive coupling between the two tuned circuitsdue to mere adjacency is sufficient to provide the decoupling signal.This is difficult to achieve however, because physical rearrangement ofthe circuits simultaneously alters the mutual coupling Z_(M), therebymaking the balanced position difficult or impossible to find.

The reactance decoupling system using an inductance is superior to thecapacitance in terms of its frequency-tracking ability. When the rffrequency is in the vicinity of the resonant frequency of LC1 (see FIG.3), the decoupling signal fed to LC2 by Z_(D) must be of the correctamplitude and phase to cancel the mutual inductance Z_(M). Similarly,when the rf frequency is in the vicinity of the different resonantfrequency of LC2 the reactance element must continue to feed back thecorrect amplitude and phase to cancel the mutual inductance Z_(M). Whenan inductance L10 is used for reactance Z_(M), the decoupling signalamplitude tracks the inductively coupled mutual coupling Z_(M) sinceboth transfers are by way of inductance. However, when Z_(D) is acapacitance, the change in amplitude of the decoupling signal withchange in frequency is opposite to the change in amplitude due toinductive mutual coupling with change in frequency. Thus, gooddecoupling can only be achieved at one resonant frequency withcapacitance decoupling.

Whenever the rf frequency is swept past the resonant frequency of one ofthe tuned circuits in the keying circuit 10, the tuned circuit absorbs agreater amount of rf energy than at other times. If the frequency atwhich this increased absorption occurs coincides with the frequency towhich either tuned detectors 34 or 36 is tuned, the respective tuneddetector 34 or 36 enables one input of coincidence gate 38. If theresonant frequency of the second tuned circuit in the keying circuit 10coincides with the frequency to which the second tuned detector 34 or 36is tuned, the respective tuned detector 34 or 36 enables the secondinput to coincidence gate 38. Both inputs to coincidence gate 38 beingenabled by the presence of correctly tuned keying circuit 10, thecoincidence gate 38 connects an enable signal in parallel to one inputof each of the two output gates 42, 44.

A dead oscillator detector 40 receives samples of the rf energy fromboth oscillators 18, 22. If an oscillator is dead, or if its energy issubstantially absorbed over the entire sweep frequency range by anabsorbent material such as iron, the dead oscillator detector 40provides an inhibit signal to one input of each of the two output gates42, 44. If the deck lock control oscillator 18, which is the oscillatorgated on in this discussion, contains substantially full rf energyexcept at a few resonant absorption points, the dead oscillator detector40 provides an enable signal to one input of the two output gates 42,44.

The third input to output gate 44 is enabled at this time by the signalfrom the sequencing generator 28 which enable the deck lock controloscillator 18. For example, when the deck enable signal 46 is connectedto deck lock control oscillator 18, it is also connected to the thirdinput of deck output gate 44. The deck output gate 44 produces a deckunlock signal 16 for connection to an electrically operated deck lock(not shown). At this time, the door output gate 42 is inhibited by theaternative output of the sequencing generator 28. Thus, lock control atthis time is restricted to the deck unlock signal 16.

The preceding completes the single-channel functional description of thedeck-lock-control portion of the system. The following paragraphoutlines the differences in the door-lock-control portion of the system.

At the next alternation of the sequencing generator 28 output, the decksignal 46 is replaced by an inhibit signal and a door enable signal 48is connected to the door lock control oscillator 22 and to the dooroutput gate 42. The first door control tank circuit L1-C2 is located inthe vicinity of one door; a second door control tank circuit L2-C3 islocated in the vicinity of a second door. Both door control tankcircuits are fed swept rf energy in parallel by the door lock controloscillator 22. When a correct keying circuit 10 is coupled to eitherdoor control tank circuit L1-C2 or L2-C3, radio frequency energy isabsorbed at two keying frequencies are previously described. The firstand second detectors 34 and 36 detect the energy depletion at the keyingfrequencies, enable door output gate 42, and produce a door lock controloutput 14 in a manner analogous to the production of the deck unlockoutput 16 previously described.

Detailed functioning of the system is described with reference to theschematic diagram shown in FIG. 2. Each function previously identifiedis boxed and identically numbered in this drawing. The deck lock controloscillator 18, with its associated circuits is identical to the doorlock control oscillator 22 with its associated circuits. Consequently,only the operation of the deck lock control circuits will be describedin detail. Functional differences between the two control circuits willbe described at the end of the detailed single-channel description.

The deck lock control oscillator 18 is an oscillator made up oftransistor Q5 and associated components. A capacitive divider made up ofcapacitors C12 and C13 provide positive feedback from emitter to base ofQ5 to sustain oscillation.

When the positive gating voltage from the sequencing generator 28appears at the collector of Q5, a ground inhibit signal simultaneouslyconnected to the collector of Q4 in the door lock control oscillator 22.Current through R5 and L4 flows through forward-biased rf bypass diodeD5 in the door lock control oscillator 22. Forward biased diode D5provides a short-circuit path to ground for rf energy from the dooroscillator tank circuits through bypass capacitor C6. This rf bypasseffectively places an rf ground at the junction of varactor diode D1 andthe two door tank circuits L1, C2 and L2, C3. This rf bypass preventsthe door tank circuits from interacting with the deck lock controloscillator 18 during its operation. The positive gating voltage at thecollector of Q5 back-biases rf bypass diode D6 in the deck lock controloscillator 18. With rf bypass diode D6 back biased, bypass capacitor C7is ineffective to shunt rf energy to ground. Rf choke L5 isolates the rfin Q5 from the bias voltage source. Thus Q5 is enabled to generate rfenergy.

The sequencing generator 28 is made up of amplifiers A5, A6 and A7 withfrequency-determining feedback components R21 and C35. The output ofamplifier A6 is a square wave alternating between zero volts andpositive voltage. The output of amplifier A6 is connected to door outputgate 42 and to door lock control oscillator 22. Inverter amplifier A7,also receiving the output of amplifier A6, provides an output which isthe inverse of its input. For example, when the output of amplifier A6is zero volts, the output of inverter amplifier A7 is positive, and viceversa. The output of inverter amplifier A7 is connected to deck outputgate 44 and to deck lock control oscillator 18. It will be evident that,whenever the deck lock control oscillator 18 and its associated deckoutput gate 44 are enabled by the positive output of inverter A7, thezero output of amplifier A6 must inhibit both door lock controloscillator 22 and its associated door output gate 42.

The positive voltage at the collector of oscillator transistor Q5 backbiases diode D6 thus removing the ac short circuit between base andcollector of Q5 through C7 and previously conducting diode D6.Oscillator transistor Q5 begins generating rf energy at a frequencydetermined by its tank circuit L3, C4, cable 20 impedance, straycapacitance, and the sweep voltage across varactor diode D2 generated bysweep generator 30.

The sweep generator consists of an integrating capacitor C1, a chargingcurrent source transistor Q1 and a switch Q2, Q3. Assume initially thatswitch transistors Q2 and Q3 are turned off and integrating capacitor C1is discharged. The voltage divider consisting of resistors R2 and R4holds the base of switch transistor Q2 at approximately 2.5 volts. Theemitter of Q2 is initially at zero volts due to the discharged conditionof C1. The emitter-base junction of Q2 is consequently held in theback-biased condition as long as its base voltage remains more positivethan its emitter voltage.

Integrating capacitor C1 begins to charge from the positive supplythrough limiting resistor R1 and the emitter-collector junction ofcurrent supply transistor Q1. The approximately linear voltage increasein integrating capacitor C1 is connected in parallel to sweep varactordiodes D1 and D2 in the tank circuits of door lock control oscillator 22and deck lock control oscillator 18, respectively. When the voltageacross the integrating capacitor reaches 3.15 volts (2.5 volts bias +0.65 volt base-emitter drop), transistor Q2 is turned on. The positivevoltage now appearing at the base of transistor Q3 causes Q3 to alsoturn on. The current in the emitter-collector path of Q3 increases thevoltage drop across resistor R2 to approximately 7.35 volts. Thisvoltage drop holds the base of Q2 at 0.65 volts as long as currentcontinues to flow in Q3. Integrating capacitor C1 is rapidly dischargedthrough the emitter-collector junction of Q2 and the base-emitterjunction of Q3. As soon as the charge in integrating capacitor C1 isdepleted to approximately 0.65 volts, the emitter of Q2 no longer beingmore positive than its base causes Q2 to turn off. This, in turn,removes the control voltage from the base of Q3. Q3 consequently turnsoff. The current through Q3 now being terminated causes the junction ofvoltage divider R2 and R4 to again rise to 2.5 volts. The charging ofintegrating capacitor C1 resumes. This continuing pattern ofapproximately linear charge followed by relatively instantaneousdischarge produces a sawtooth waveform which is used to sweep theoscillator 18 or 22 frequency.

Varactor diode D2 is connected in series to ground with integratingcapacitor C1. The varactor/integrator combination, D1/C1, is connectedin parallel with the deck tank circuit L3, C4. Changes in the junctioncapacitance of varactor diode D2 are therefore effective to vary thefrequency of the deck lock control oscillator 18.

A sample of the rf energy in the deck lock control oscillator 18, takenat the junction of capacitors C7 and C11, is connected to a firstcapacitive voltage divide consisting of fixed capacitor C19 and variablecapacitor C25, and to a second capacitive voltage divider consisting offixed capacitor C20 and variable capacitor C27. The two capacitivevoltage dividers are adjusted after installation to compensate for thefact that the amplitude of the rf energy generated by Q5 varies acrossthe sweep frequency range. Typically, rf energy is lower at thelow-frequency end of the sweep. When correctly adjusted, the ac signalcoupled to first detector 34 at frequency F1 equals the ac signalcoupled to second detector 38 at frequency F2. In addition, adjustmentof the capacitive voltage dividers from deck lock control oscillator 18plus a corresponding pair of capacitive voltage dividers C18, C24 andC17, C22 from door lock control oscillator 22 compensate for rf energydifferences between the two oscillators.

Within first detector 34, capacitor C26 couples the rf energy from thejunction of capacitive voltage divider C19, C25 to a sharplyparallel-resonant circuit comprised of inductor L6 and capacitor C29.This resonant circuit is tuned to the first keying frequency. In theabsence of a keying circuit 10, each time the oscillator frequency isswept past the first keying frequency, the rf voltage across L6 and C 29is increased by the Q of the resonant circuit. An rf voltage spike isthus generated each time the frequency is swept past the first keyingfrequency. This rf voltage spike is detected by diode D8 which connectsthe envelope of the rf spike to the base of amplifier transistor Q7. Thepositive base voltage turns off transistor Q7. The resulting low inputto inverter amplifier A1 causes inverter amplifier A1 to generate asequence of positive output pulses. Diode D10 feeds the positive pulsesinto peak-detector capacitor C32. The time constant of peak-detectorcapacitor C32 and bleeder resistor R18 is such that if one rf spike isdetected per frequency sweep, peak-detector capacitor C32 remainssufficiently charged to maintain the output of inverter amplifier A2 atapproximately zero volts. The resulting zero-volts output of inverter A2inhibits one input of each of output gates 42 and 44. Thus if only thecircuit tuned to the first keying frequency in keying circuit 10 isabsent, the result is complete denial of a control output regardless thepresence or absence of other tuned circuits in the keying circuit 10.

When a resonant circuit C37, L8 or C38, L9, tuned to the first keyingfrequency, is inductively coupled to the deck sensing coil L3, the rfenergy at the first keying frequency is depleted by absorption in thekeying circuit. Thus, as the oscillator frequency is swept past thefirst keying frequency, the parallel-resonant circuit C29, L6 in thefirst detector 34 finds insufficient rf energy with which to form an rfspike. Consequently, no energy is stored in peak-detector capacitor C32as the result of an rf spike at frequency F1.

Second detector 36 operates in the same fashion as just described forfirst detector 34. If a properly tuned circuit in the keying circuitalso absorbs energy at frequency F2, the rf spike otherwise generated byL7 and C31 is suppressed in the same manner as described for thesuppression of the F1 spike. With both rf spikes suppressed,peak-detector capacitor C32 discharges through bleeder resistor R18. Assoon as the voltage across peak-detector capacitor C32 approaches zero,the output of inverter amplifier A2 switches from zero volts to apositive enable signal. This positive enable signal enables one input ofdoor output gates 42 and deck output gate 44.

A second input to the deck output gate 44 is provided by a signal fromdead oscillator detector 40 which is generated as described in thefollowing sentences. A sample of the rf energy in the deck lock controloscillator 18 is rectified in diode D4 and connected as a sequence ofnegative half cycles through capacitor C15 to the base of transistor Q6.With the values given for capacitor C15 and C14 and resistor R10,transistor Q6 is unable to respond at the rf frequency. If no tunedcircuit is coupled to the sensing coil L3, or if deck lock controloscillator 18 is dead, transistor Q6 produces a null output. CapacitorC16, failing to receive charging signals from transistor Q6 remainsdischarged by bleeder resistor R13. The resulting zero-volt signalinhibits one input of door output gate 42 and deck output gate 44. Thus,if an alternating component in the rf envelope is not produced by thepresence of a tuned keying circuit, the output gates 42, 44 remaininhibited. The absence of the alternating component in the rf envelopemay be due to the absence of a tuned circuit, the non-functioning of theoscillator, or to the presence of an absorber, such as iron whichabsorbs the rf energy at all frequencies.

If any resonant circuit, tuned within the sweep range of the functioningdeck lock control oscillator 18 is coupled to the sensing coil (whetheror not the resonant frequency matches frequency F1 or F2), the resultingamplitude-modulated component in the rf envelope causes transistor Q6,normally turned on, to be turned off momentarily each time theoscillator frequency sweeps past the frequency of the external resonantcircuit. The resulting positive alternations in the output of transistorQ6 are connected through diode D7 to capacitor C16. Capacitor C16becomes charged to approximately the peak of the positive-going signalat the collector of transistor Q6. The resistance of bleeder resistorR13 is so high that, as long as positive charging signals occur at thesweep rate, it does not significantly deplete the charge in capacitorC16. The positive voltage stored in C16 provides the enable signal whichenables the second input to deck output gate 44.

The third input to deck output gate 44 is enabled, as previouslydescribed, by the high output assumed at this time from inverter A7 inthe sequencing generator 28. A leading-edge delay circuit composed ofresistor R19 and capacitor C33 on the input to deck output gate 44applies a few milliseconds delay to the onset of the gating signal fromsequencing generator 28 to ensure that the peak-detector capacitor C32is given time to charge and dead-oscillator detector 40 capacitor C16 isgiven time to discharge following the end of the preceding door cycle.Without the slight delay imposed in this way, if a door control signalis properly generated in the preceding time period, the initiation ofthe deck control time period finds capacitor C32 fully discharged andC16 fully charged. Since it takes a few frequency sweeps to fully chargecapacitor C32 and discharge capacitor C16, an immediate application ofthe sequence generator 28 signal to the deck output gate 44 wouldproduce an undesired unlock signal. The delay imposed by theleading-edge delay circuit R19, C33 avoids such undesired unlocksignals.

When all inputs to NAND gate G2 in deck output gate 44 are enabled, theresulting low output is amplified and inverted in inverter A4 andconnected through R26 to the base of output control transistor Q11.Output control transistor Q11 is turned on by the positive voltage atits base. The resulting reduced voltage at the base of output transistorQ12 turns output transistor Q12 on. The emitter-collector junction ofoutput transistor Q12 provides a positive control output signal 16 foroperation of the deck lock (not shown).

The preceding completes the detailed single-channel description of thedeck lock control portion of the system. The following paragraphs detailthe differences to be found in the operation of the door lock controlportion of the system. Description of those functions which are the samein the two portions of the system is omitted.

At the end of the deck control time period, the outputs of thesequencing generator 28 are reversed. The positive enable signal,previously connected from inverter A7 in the sequencing generator 28 totransistor A5, is replaced by a ground signal. The ground signalpreviously connected from amplifier A6 in the sequencing generator 28 totransistor Q4, is replaced by a positive enable signal. The groundsignal at the collector of Q5 turns off the deck lock control oscillator18 and causes rf bypass diode D6 to become forward biased.Forward-biased diode D6 acts as an rf short from the deck tank circuitL3, C4 through bypass capacitor C7 to ground. This rf bypass patheliminates interaction between the deck control tank circuit L3, C4 andthe active door lock control channel.

The door lock control channel contains two tank circuits L1, C2 and L2,C3 which are fed rf energy in parallel rather than the single tankcircuit L3, C4 as described for the deck lock control channel. Althoughcircuit values are adjusted slightly to ensure that the full frequencysweep is attainable, the operation of the front end of the door lockcontrol channel is otherwise identical to the deck lock control channel.

The door output gate 42 is similar to the deck output gate 44 except forthe substitution of a darlington output amplifier, Q9, Q10, in place ofthe single-transistor output amplifier Q12 used in the deck output gate44. The higher gain obtainable with the darlington output amplifier Q9,Q10 is necessary to produce a door lock control signal 14 capable ofsimultaneously operating the locks on both doors instead of thesingle-lock operation required by the deck lock control channel.

The following list of circuit component values and identities areillustrative of one practical embodiment of the invention. It will bereadily evident to one skilled in the art that different componentvalues or arrangements will produce equivalently functioning systemswithout departing from the teachings of the invention.

    ______________________________________                                        Resistances                                                                             Capacitances                                                        (ohms)    (microfarads)     Transistors                                       R1   22K      C1      .01         Q1   2N4248                                 R2   22K      C2      20-500 pf (shielded                                                                       Q2   2N4248                                                       cable capacitance)                                      R3   1M       C3         "        Q3   2N5132                                 R4   10K      C4         "        Q4   2N5132                                 R5   33K      C5      .01         Q5   2N5132                                 R6   1.5M     C6      .01         Q6   2N5132                                 R7   10K      C7      .01         Q7   2N4248                                 R8   1.5M     C8      200pf       Q8   2N3567                                 R9   10K      C9      200pf       Q9   MJE371                                 R10  470K     C10     200pf       Q10  2N3055                                 R11  3.3M     C11     200pf       Q11  2N3567                                 R12  220K     C12     200pf       Q12  MJE371                                 R13  10M      C13     200pf       Integrated                                  R14  270K     C14     .001        Circuits                                    R15  10M      C15     470pf       A1   CD4009AE                               R16  470K     C16     .027        A2   CD4009AE                               R17  not used C17     15pf        A3   CD4009AE                               R18  1.5M     C18     15pf        A4   CD4009AE                               R19  1M       C19     15pf        A5   CD4009AE                               R20  1M       C20     15pf        A6   CD4009AE                               R21  1.5M     C21     10pf        A7   CD4023AE                               R23  10K      C22     5-30 pf var Gates                                       R24  470      C23     10pf        G1   CD4023AE                               R25  10K      C24     5-30pf var  G2   CD4023AE                               R26  10K      C25     5-30pf var                                              R27  470      C26     10pf        Diodes                                      R28  10K      C27     5-30pf var  D1   MV1401                                 R29  10K      C28     10pf        D2   MV1401                                               C29     10-180pf var                                                                              D3   IN4148                                               C30     .0047       D4   IN4148                                               C31     10-180pf var                                                                              D5   IN4148                                               C32     .068        D6   IN4148                                               C33     .01         D7   IN4148                                               C34     .01         D8   IN4148                                               C35     .22         D9   IN4148                                               C36     not used    D10  IN4148                                               C37     56pf        D11  IN5060                                               C38     50pf                                                    Inductances                                                                   (microhenry)                                                                  L1   39                                                                       L2   39                                                                       L3   39                                                                       L4   1500                                                                     L5   39                                                                       L6   5                                                                        L7   5                                                                        L8   10.5                                                                     L9   10.5                                                                     L10  3.5                                                                      ______________________________________                                    

It will be understood that the claims are intended to cover all changesand modifications of the preferred embodiments of the invention, hereinchosen for the purpose of illustration which do not constitutedepartures from the spirit and scope of the invention.

What is claimed is:
 1. In a dual channel keyable control circuit of thetype having swept rf energy coupled to at least one sensing coil, theinvention of a keying circuit to trigger said keyable control circuitinto producing a control output signal comprising:a. a first resonantcircuit containing at least a first inductor in parallel with a firstcapacitor said first resonant circuit being resonant at a first rffrequency; b. a second resonant circuit containing at least a secondinductor in parallel with a second capacitor said second resonantcircuit being resonant at a second rf frequency different from saidfirst rf frequency; c. a common connection between said first and secondresonant circuits; d. a reactance in the common connection between saidfirst and second resonant circuits; e. when said swept rf energy is atthe resonant frequency of said first resonant circuit, said reactancefeeding into said first resonant circuit a signal of substantiallycorrect amplitude and phase to cancel the rf energy coupled into saidfirst resonant circuit by the proximity of said second resonant circuit;and f. when said swept rf energy is at the rf frequency of said secondresonant circuit, said reactance feeding into said second resonantcircuit a signal of substantially correct amplitude and phase to cancelthe rf energy coupled into said second resonant circuit by the proximityof said first resonant circuit.
 2. A keying circuit as recited in claim1 wherein said reactance is a capacitance.
 3. A keying circuit asrecited in claim 2 wherein said first and second inductors are wound inopposite directions.
 4. A keying circuit as recited in claim 1 whereinsaid reactance is an inductance.
 5. A keying circuit as recited in claim4 wherein said first and second inductors are wound in the samedirection.